Because they can be supported by faster buses, which will be able to operate the RAM at higher frequencies. Just because such chipsets don't exist now, we cannot infer that they will never exist. However, it's possible to use the RAM modules on existing systems with slower buses. This means that users can buy the RAM now to use on current chipsets, and when they upgrade to a faster bus speed the RAM will already be able to take advantage of it.

Whenever two things have to work together there's a chicken-and-egg problem associated with making improvements. Unless you can make both things better together (say you run a factory making both nuts and bolts), there will likely be times when one side of the operation has a technological lead over the other, but both need to have the improvement before anyone gets an advantage. If you support backwards compatibility, you can get your improved version straight out to market before the new supporting products are ready.

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